An MRI apparatus is an apparatus which magnetically excites nuclear spins in a subject placed in a static magnetic field, with the use of a Radio Frequency (RF) pulse having Larmor frequency, and then reconstructs an image showing internal structures of the subject based on magnetic resonance signals generated by the excitation. In the related art, there is a method of imaging blood flow by using the MRI apparatus (for example, refer to JP-A 2009-56072 (KOKAI)).
For example, there is an Arterial Spin Labeling (ASL) as an example of a method of imaging blood flow in a non-contrast-enhanced manner (for example, refer to (1) Edelmann R R at al., Radiology 192: 513-519 (1994); (2) KIMURA Tokunori, “non-invasive blood flow imaging according to Modified STAR using asymmetric inversion slabs (ASTAR) method” Journal of Japanese Society of Magnetic Resonance in Medicine, 2001, 20 (8), 374-385; (3) Kwong K K, Chesler D A, koff R M, Donahue K M, et al., “MR perfusion studies with T1-weighted echo planar imaging”, MRM (Mag. Reson. Med), 34: 878-887 (1995); (4) Dixon W T et al., MRM, 18: 257 (1991); and (5) Non-enhanced Time-Resolved MRA using Inflow Arterial Spin Labeling, 2009 ISMRM, pp 3486). In general, in the ASL, the MRI apparatus generates an image of only blood flow components, where stationary tissues are erased, by generating subtraction images between tag images obtained in a tag mode and control images obtained in a control mode.
The tag mode referred herein is, for example, an imaging mode of applying an RF wave to an upstream portion of the artery passing through an imaging area to impart a label referred to as a tag to blood being flown into the imaging area and, and performing imaging after a lapse of an inversion time (TI) after a predetermined label after application of the RF wave. In addition, the control mode is an imaging mode of, after a lapse of a predetermined TI, performing the magnetic resonance data collection without performing the fluid labeling through application of the RF wave to the upstream portion of the imaging area. In other words, the control mode is the imaging mode other than the tag mode among non-contrast-enhanced MRA (MR Angiography) imaging modes. As the control mode, there are, for example, a control mode of performing non-contrast-enhanced imaging without performing imparting a tag to a fluid, a control mode of imparting a tag to a fluid in an imaging area, a control mode of imparting a tag to a fluid in a downstream portion of the imaging area, or the like.
In addition, there is a method of generating an image of the behavior of a blood flow by repetitively performing ASL with TI being changed. In this method, the MRI apparatus generates an image pair composed of the tag image and the control image for each TI and generates a subtraction image including only a blood flow component for each TI. Hereinafter, a method of collecting an image pair composed of a tag image and a control image for each TI and generating a subtraction image of each image is referred to as an “N−N subtraction method.”
In addition, there is a method of generating an image of a blood flow by using only the tag image without generating the subtraction image between the tag image and the control image. For example, there is a method referred as a Multiple IR (mIR) method (for example, refer to Non-enhanced Time-Resolved MRA using Inflow Arterial Spin Labeling, 2009 ISMRM, pp 3487; Quantitative Dynamic MR Angiography using ASL based True FISP., 2009 ISMRM, pp 3635; and Mani S et al., MRM, 37: 898-905 (1997)). In this method, the MRI apparatus applies an area-selective saturation pulse to the imaging area, and after that, an area-non-selective inversion recovery (IR) pulse several times. Next, the MRI apparatus generates a blood flow image, where the signal intensity in the stationary tissue is suppressed, by starting the magnetic resonance data collection at the time where the longitudinal magnetization of the stationary tissue is recovered from a negative value to near zero due to the longitudinal relaxation. Hereinafter, as an mIR method, a method of obtaining a blood flow image without generation of a subtraction image is referred to as an “mIR subtraction-less method.”
In addition, a method of simultaneously using the N−N subtraction method and the mIR method is also disclosed (for example, refer to Mani S et al., MRM, 37: 898-905 (1997)). In this method, the MRI apparatus generates a tag image and a control image by using the mIR method and generates a subtraction image between the tag image and the control image. Hereinafter, this method is referred to as an “mIR N−N subtraction method.”
The N−N subtraction method is advantageous in that the stationary tissue can be erased with high accuracy but it is problematic in that the imaging time is long. FIG. 15 is a view illustrating a change in signal in association with TI in a known N−N subtraction method. In FIG. 15, the longitudinal axis represents a signal intensity (Stag) of a tag image, and the transverse axis represents TI. As illustrated in FIG. 15, in the N−N subtraction method, the signal intensity (Sstationary illustrated in FIG. 15) in the stationary tissue in the tag image varies according to a change in TI. In addition, similarly to the tag image, the signal intensity in the control image also varies. Therefore, in order to erase the stationary tissue with high accuracy, it is necessary to generate an image pair composed of a tag image and a control image for each TI and to generate a subtraction image for each TI. Accordingly, in the N−N subtraction method, it is necessary to perform two times of data collection (the data collection for the tag image and the data collection for the control image) for the same TI. As a result, the imaging time increases.
In addition, in a conventional mIR subtraction-less method, the imaging time for generating a blood flow image is relatively short because only the tag image used. However, this method is problematic in that it is difficult to adjust the number of applications of the area-non-selective IR pulse and the timing of starting the data collection in order to erase the stationary tissue with high accuracy. In general, the stationary tissue in the imaging area includes plural types of tissues such as fat, cerebrospinal fluid, white matter, and gray matter. However, the longitudinal relaxation time (T1) indicating a time interval from the time of excitation due to the application of an RF wave to the time of recovery to a steady state, is different according to the type of tissue. Therefore, it is difficult to adjust the number of applications of the area-non-selective IR pulse and the timing of starting the data collection so that, for all types of tissues, the time where the longitudinal magnetization is recovered to near zero is coincident with each other. For example, in the case where the number of applications of the area-non-selective IR pulse is set to two, the tissue such as fat, of which the T1 value is short, may remain. In addition, although the number of applications of the area-non-selective IR pulse is set to two and the signal intensity of the tissue, of which the T1 value is short, may be allowed to be near zero, in this case, the signal intensity of the tissue such as cerebrospinal fluid, of which the TI value is long, cannot be allowed to be near zero. In addition, although the signal intensity of the plural types of tissues may be allowed to be near zero by increasing the number of applications of the area-non-selective IR pulse up to three or more, in this case, the imaging time is increased. In this manner, in the mIR subtraction-less method, although the number of applications of the area-non-selective IR pulse and the timing of starting the data collection are adjusted so that the stationary tissue may be erased with high accuracy, there is a limitation in suppressing the background.
In addition, the aforementioned problems occur not only in the case where the blood flow image is imaged but occur also in the case of imaging other fluids (for example, cerebrospinal fluid, or the like).